Unveiling the photocatalytic potential of graphitic carbon nitride (g-C3N4): a state-of-the-art review

Graphitic carbon nitride (g-C3N4)-based materials have emerged as promising photocatalysts due to their unique band structure, excellent stability, and environmental friendliness. This review provides a comprehensive and in-depth analysis of the current state of research on g-C3N4-based photocatalysts. The review summarizes several strategies to improve the photocatalytic performance of pristine g-C3N4, e.g., by creating heterojunctions, doping with non-metallic and metallic materials, co-catalyst loading, tuning catalyst morphology, metal deposition, and nitrogen-defect engineering. The review also highlights the various characterization techniques employed to elucidate the structural and physicochemical features of g-C3N4-based catalysts, as well as their applications of in photocatalytic degradation and hydrogen production, emphasizing their remarkable performance in pollutants' removal and clean energy generation. Furthermore, this review article investigates the effect of operational parameters on the catalytic activity and efficiency of g-C3N4-based catalysts, shedding light on the key factors that influence their performance. The review also provides insights into the photocatalytic pathways and reaction mechanisms involving g-C3N4 based photocatalysts. The review also identifies the research gaps and challenges in the field and presents prospects for the development and utilization of g-C3N4-based photocatalysts. Overall, this comprehensive review provides valuable insights into the synthesis, characterization, applications, and prospects of g-C3N4-based photocatalysts, offering guidance for future research and technological advancements in this rapidly growing field.


Introduction
The discharge of pollutants into aquatic environments has increased signicantly as a result of the manufacturing sector's growth. 1,2Because of their hazardous characteristics and possible carcinogenicity, organic pollutants found in both air and water are especially concerning. 3The chemical processing Mahmoud A: Ahmed Mahmoud Adel Ahmed earned his PhD degree in 2024.He has been actively engaged in research for the past eight years and his research focuses on the synthesis, characterization, and environmental applications of nanomaterials and their composites in water treatment and remediation.He has authored several reviews and book chapters on these topics.He also serves as a senior service engineer at Veolia Environmental Services, managing various sectors like reverse osmosis, boilers, cooling towers, and wastewater plants.industries, building materials, textile production, and coatings used in indoor furniture are the main producers of these pollutants. 4,5][6] Furthermore, the mutagenic and carcinogenic impacts of these pollutants are noteworthy. 7,8Moreover, organic pollutants, such as dyes, pesticides, pharmaceuticals, phenols, and others, signicantly impact the receiving water bodies by changing key variables like unpleasant odors, color, toxicity levels, biochemical oxygen demand (BOD), and chemical oxygen demand (COD).Some of these organic pollutants have long half-life times, (bio)accumulate, are not easily degraded, and damage the marine ora and fauna, aquatic lives, and ultimately human health.In addition to these environmental concerns, the global community also faces a pressing challenge in terms of ensuring energy security. 9Fossil fuels are limited resources, and using them to produce energy increases harm to the atmosphere by emitting various pollutants, including carbon dioxide. 10This has spurred a global effort to explore technologies that promote the utilization of renewable energy sources and address environmental challenges. 113][14][15][16][17] However, when handling complicated pollutants with a variety of chemical and physical features, these conventional approaches have limits in terms of efficiency and energy usage, as well as the increased risk of generating secondary pollutants. 184][25][26][27] The efficient utilization of solar photocatalysis holds signicant research value in terms of improving the environment and reducing greenhouse gas emissions.Typically, a photocatalytic process involves stages, such as harnessing visible light, exciting photocarriers, segregating and migrating photo-induced charge carriers to active sites, and facilitating the redox process on the photocatalyst surface. 28,293][34] In particular, g-C 3 N 4 , a metal-free polymer semiconductor containing tri-s-triazine units, has garnered a great deal of interest due to its potential uses in photochemistry and photocatalysis. 35raphitic carbon nitride (g-C 3 N 4 ) is regarded as one of the rst organic conjugated polymers, having been discovered in 1834. 36There are ve primary phases that g-C 3 N 4 may be categorized into: the cubic phase, the pseudo-cubic phase, the graphitic phase with minimal compressibility and remarkable hardness that is comparable to a diamond, the a-phase, and the b-phase. 37Research communities have become quite excited by g-C 3 N 4 -based materials as photocatalysts because of its nontoxicity, high visible light harvesting, p-conjugated assembly, increased profusion, and chemical and thermal durability. 38,39he optical bandgap of g-C 3 N 4 at 2.7 eV (460 nm), with VB and CB potentials at −1.09 and +1.56 V (vs.NHE), respectively, make g-C 3 N 4 attractive material for overall water splitting. 40,41urthermore, the widespread usage of g-C 3 N 4 -based materials as a visible-light-driven photocatalyst is mostly due to its easy synthesis process from readily accessible, affordable precursors. 42,43Additionally, g-C 3 N 4 has a powerful electrical conductivity and distinct conjugated structure due to the graphitic stacking of g-C 3 N 4 layers connected by tertiary amines. 44,45The presence of carbon and nitrogen atoms with distinct valence states results in the creation of multiple band structures; therefore, pristine g-C 3 N 4 has shown promise as a photocatalyst, but it also has limitations that must be addressed. 43,46ne major limitation is its low photocatalytic activity, attributed to its wide bandgap energy, which limits its absorption of the solar spectrum. 47Additionally, the performance of photocatalytic techniques is further decreased by the quick coupling of photo-generated charge carriers in g-C 3 N 4 . 48It also has limited charge carrier mobility, hindering efficient charge transfer.Other limitations of pristine g-C 3 N 4 are its relatively low specic surface area and lack of stability under photocatalytic conditions, as prolonged exposure to light and reactive species can degrade its performance over time.][51][52] Heterostructure development has emerged as the most promising approach to improve the photocatalytic activity of g-Ashraf A: Mohamed Ashraf A. Mohamed is a professor of environmental analytical chemistry, at the Department of Chemistry, Faculty of Science, Ain Shams University, Cairo, Egypt.He earned his MSc degree in 1991 and his PhD degree in 1995.He has been actively engaged in research for the past 35 years and his current research interests include analytical chemistry, nanomaterials, layered double hydroxides, molecularly imprinted polymers, water treatment and analysis, optical sensors, and paper micro-uidics.He has authored several reviews and book chapters on these topics.C 3 N 4 .One of the advantageous properties of g-C 3 N 4 is its tunable band gap, which allows precise control over the energy levels of its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). 53This tunability signicantly impacts the photoelectronic performance of g-C 3 N 4 as a photocatalytic nanosheet.By constructing hetero structures, the band gap of g-C 3 N 4 can be effectively modied, leading to expanded light harvesting and promoting the separation of hole-electron pairs. 54,55This modication approach involves the intentional introduction of metal, nonmetal, or other nanomaterials into the structure, offering a means to enhance the photocatalytic performance of g-C 3 N 4 .
7][58][59] Moreover, there is increasing research interest in utilizing g-C 3 N 4composites for hydrogen generation. 60The number of publications focusing on pollutant removal and H 2 -production using g-C 3 N 4 nanocomposites has shown a notable increase over the last few years, as seen in Fig. 1.Initially, there were only a few publications per year, indicating limited attention to the topic.However, since 2017, there has been a rapid upward trend in both citations and publications, signifying a growing interest in the eld, where documents on photocatalysis were almost ve times higher than those on H 2 -production.Most of these publications consist of journal articles (93%), with a smaller fraction being reviews (4.9%), and conference articles (1.1%), as shown in Fig. 1.This indicates a scarcity of dedicated and updated review papers, which are essential for providing interested researchers and the scientic community with a comprehensive and up-to-date evaluation of g-C 3 N 4 -composites' application as photocatalysts.
This comprehensive review aims at providing a detailed examination of the synthesis methods of g-C 3 N 4 -based photocatalysts, along with their applications in environmental remediation, e.g., organic pollutants' degradation and hydrogen production.Additionally, the review highlights the characterization techniques used to understand the crystal structure, morphology, surface area, nanoparticle distribution, and compositional properties of g-C 3 N 4 -based photocatalysts.Moreover, the review describes the mechanisms and factors inuencing the photocatalytic performance of g-C 3 N 4 -based photocatalysts in organic pollutant degradation, providing insights into the identication of key intermediates and reactive species involved in the photocatalytic degradation processes.It further investigates the strategies employed to enhance the efficiency and selectivity of g-C 3 N 4 -based photocatalysts, including the utilization of metal cocatalysts, co-doping techniques, heterojunction formation, and surface modication.Additionally, the review assesses the g-C 3 N 4 -based photocatalysts' application in hydrogen production through water splitting, evaluating their performance in terms of hydrogen evolution rate, stability, and selectivity, while discussing the underlying mechanisms of photogenerated charge separation and transfer.
2 Modification of g-C 3 N 4 for improved photocatalytic activity Composite g-C 3 N 4 photocatalysts have gained signicant attention in recent years due to their potential for efficient and sustainable energy conversion and environmental remediation.The g-C 3 N 4 modication with other materials allows for improved light absorption, better charge separation, and boosted catalytic performance, resulting in enhanced photocatalytic activity.
Several approaches have been applied to modify pristine graphitic carbon nitride and improve its photocatalytic performance, such as creating heterojunctions, doping with nonmetallic and metallic materials, co-catalyst loading, tuning catalyst morphology, metal deposition, and nitrogen-defect engineering, as shown in Scheme 1. [49][50][51][52]61,62 When it comes to the fabrication of g-C 3 N 4 composites as photocatalysts, two main approaches are commonly employed based on the crystallization process: in situ crystallization and ex situ crystallization.

Synthesis of g-C 3 N 4 composites by in situ crystallization
In situ crystallization: the g-C 3 N 4 composite is fabricated by incorporating the other material during the polymerization process of g-C 3 N 4 itself.This approach involves the cocondensation of a precursor monomer of g-C 3 N 4 with other components, which subsequently polymerizes and crystallizes simultaneously. 63,64During the in situ crystallization process, the precursor monomers of g-C 3 N 4 , typically urea, thiourea, melamine, cyanamide, or dicyanamide are combined with the desired components, such as metal precursors or carbon-based materials.The mixture is then subjected to thermal treatment under specic temperature and atmosphere conditions.The heating process triggers the polymerization and condensation of the monomers into a layered g-C 3 N 4 structure, thereby incorporating the additional components into the composite.In situ crystallization offers several advantages, including uniform distribution of the composite components and good interfacial interaction between g-C 3 N 4 and the additional material.This approach allows for control over the composition and structure of the composite, leading to improved photocatalytic performance. 65,66

Synthesis of g-C 3 N 4 composites by ex situ crystallization
In ex situ Crystallization, g-C 3 N 4 is synthesized separately, and subsequently, other materials are introduced or deposited onto its surface to form the composite. 67To fabricate the ex situ composite, various methods can be utilized.For example, metal nanoparticles or metal oxide precursors can be deposited onto the surface of pre-prepared g-C 3 N 4 through methods like impregnation, photo-deposition, or chemical reduction.Carbon-based materials, such as graphenes or carbon nanotubes, can also be integrated with pre-formed g-C 3 N 4 through solution mixing or deposition techniques.Ex situ crystallization offers advantages such as precise control over the loading amount and distribution of the additional material.It allows for exibility in choosing the post-treatment conditions for efficient deposition or integration of the composite components, resulting in improved photocatalytic performance.The choice between in situ and ex situ crystallization depends on the specic composite design, the compatibility of the materials, and the desired properties.In situ crystallization allows for simultaneous formation of the g-C 3 N 4 composite during the polymerization process, while ex situ crystallization offers exibility in introducing and controlling the deposition of other materials onto pre-formed g-C 3 N 4 . 68

Modication of g-C 3 N 4 by metal-deposition
Metal deposition involves the introduction of metal nanoparticles or tiny thin lms onto the surface of g-C 3 N 4 through various deposition techniques, such as physical vapor deposition or chemical methods (e.g., impregnation, electrochemical deposition). 69In this process, the metal species are not incorporated into the lattice structure of g-C 3 N 4 but rather exist as separate entities on the surface.The incorporation of metals onto g-C 3 N 4 as a composite photocatalyst offers critical prospects for improving its light absorption, charge separation, catalytic activity, and overall photocatalytic performance.The localized surface plasmon resonances, catalytic properties, and synergistic effects of noble metals contribute to the enhanced efficiency and selectivity of photocatalytic reactions.For instance, a facile immobilization of noble metals (Ag, Au, and Pd) onto g-C 3 N 4 using a simple ultrasonication technique was described. 70In this method, g-C 3 N 4 (0.5 g) was dispersed in DI water through ultrasonication for 1 hour.The metal precursor was then mixed with the previous suspension, followed by reduction using NaBH 4 with continuous stirring for 1 hour.Aer noble metals' deposition, XRD examination showed a modest drop in the diffraction intensity of the g-C 3 N 4 (100) plane.This implies that the presence of metal atoms prevented the formation of g-C 3 N 4 crystals. 70Furthermore, Ag/g-C 3 N 4 photocatalyst was synthesized by using an infrared-assisted Scheme 1 Modification methods of g-C 3 N 4 to enhance its photocatalytic performance.
heating strategy to deposit AgNO 3 salt onto the g-C 3 N 4 .The presence of Ag nanoparticles on the surface of g-C 3 N 4 facilitates the capture of electrons generated by g-C 3 N 4 and their subsequent utilization in degrading methyl orange or producing H 2 from H + . 71In another investigation, researchers employed ultrasonication-assisted liquid exfoliation to create g-C 3 N 4 nanosheets from bulk g-C 3 N 4 . 72Aer that Au was deposited on g-C 3 N 4 via green photoreduction of Au(III).TEM analysis veried the good exfoliation of bulk g-C 3 N 4 (Fig. 2a).However, numerous Au NPs ranging from 5 to 20 nm were formed on the nanosheets, as depicted in (Fig. 2b).Additionally, DRS results demonstrated that the Au NPs/g-C 3 N 4 composite exhibited an absorption peak at 550 nm, indicative of the surface plasmon resonance band specic to colloidal gold (Fig. 2c).Hence, the presence of Au NPs served as electron sinks, facilitating the separation of photogenerated electron/hole pairs. 72Moreover, Ag NPs/g-C 3 N 4 composite was synthesized using an environmentally friendly chemical approach, as depicted in (Fig. 2d). 73he deposition of Ag NPs onto the g-C 3 N 4 surface resulted in a slight reduction in the BET surface area, as shown in (Fig. 2d).XPS analysis further conrmed the existence of metallic silver on the g-C 3 N 4 surface.Furthermore, chemical impregnation of single Pd atoms onto g-C 3 N 4 enhanced its photocatalytic activity. 75The presence of single Pd atoms and their coordination structure in the composite were conrmed using HAADF-STEM (high-angle annular dark-eld scanning transmission electron microscopy) and XAFS (X-ray absorption ne structure) analyses.The powerful interaction between the Pd-and surrounding N-atoms facilitated the production of photogenerated electrons, leading to the promotion of the photocatalytic performance of the composite. 75However, the noble metal's cost prevents its extensive use in real applications.Studies have been performed on various transition metals, including Fe, Cu, W, Zn, Mo, Zr, etc. [76][77][78][79][80] For example, the incorporation of cobalt into g-C 3 N 4 thorough a one-step thermal polycondensation approach suppressed the growth of the g-C 3 N 4 crystals and resulted in a larger specic surface area with the formation of abundant Co-N x active sites. 81It Also reduced the band gap energy and facilitated more efficient separation of photogenerated electrons and holes. 81Furthermore, the Fe/g-C 3 N 4 composites were fabricated with various initial concentrations of FeCl 3 , resulting in samples labeled FCN-0.5, FCN-1, FCN-2, and FCN-3 representing 0.5%, 1%, 2%, and 3% Fe, respectively. 74The DRS revealed an enhanced visible-light range absorption and a redshi for Fe/g-C 3 N 4 composites.As the Fe content increased, the optical band gap gradually shied to lower energy, indicating the incorporation of Fe ions into the g-C 3 N 4 lattice and altering its electronic structure.This redshi in absorption promoted the production of more electron-hole pairs under sunlight, ultimately enhancing the photocatalytic features.Additionally, the Nyquist plots illustrated clear differences in the semicircle diameter between bulk g-C 3 N 4 , pure g-C 3 N 4 , and FCN-2 nanosheets, with the FCN-2 nanosheets displaying a signicantly smaller semicircle diameter compared to the others (Fig. 2e). 74Moreover, the Co/gC 3 N 4 composite was fabricated through an in situ calcination strategy. 82Initially, 30 g of melamine was mixed with 50 mL of DI water.Subsequently, Co(NO 3 ) 2 was added to the suspension under sonication for 10 minutes, maintaining a weight ratio of 30 : 0.5.The resulting mixture was then calcined in a Muffle furnace at 550 °C for 1 hour at a heating rate of 10 °C min −1 . 82Co/g-C 3 N 4 had a surface area of 25.6 m 2 g −1 , featuring a larger amount of mesopores compared to g-C 3 N 4 (surface area: 18.2 m 2 g −1 ).The SEM image showed a mixed morphology in Co/g-C 3 N 4 , consisting of cobalt oxide grains with an irregular polygonal crystal shape and g-C 3 N 4 sheets.

Modication of g-C 3 N 4 by non-metallic and metallic doping
Doping involves introducing dopant into the lattice structure of g-C 3 N 4 by substituting carbon or nitrogen atoms with dopant atoms.This process modies the electronic structure and properties of g-C 3 N 4 by altering the band structure, charge carrier mobility, and recombination rates.Non-metal and metal doping are the two primary types of elemental doping of g-C 3 N 4 .Non-metal doping has gained signicant attention as a means to preserve the metal-free property of g-C 3 N 4 .4][85][86] This characteristic makes non-metals a suitable option for doping g-C 3 N 4 , as they do not introduce metal ions with varying chemical states, which could be affected by thermal variations.][89] A facile method was employed to synthesize metal-free boron and oxygen-doped g-C 3 N 4 with carbon vacancy. 90In this method, a mixture of g-C 3 N 4 and varying amounts of H 3 BO 3 (1%, 2.5%, 5%, and 10%) was ground and transferred to a crucible for calcination at 500 °C for 2 hours.The resulting B and O doped g-C 3 N 4 exhibited distinct morphological characteristics compared to pristine g-C 3 N 4 , featuring loose and irregular tissue-like structures.SEM images revealed that the B and O dopants caused a modication in the morphology by dividing the bulk layers of g-C 3 N 4 into smaller layers. 90hosphorus-doped g-C 3 N 4 was fabricated via a simple polycondensation strategy using dicyandiamide (or cyanoguanidine) as the precursor and 1-butyl-3-methylimidazolium hexa-uorophosphate as the phosphorus source. 91The hexauorophosphate ions reacted with amine groups upon raising the temperature, incorporating phosphorus into the C-N framework.Analysis conrmed the formation of P-N bonds, with phosphorus likely substituting corner or bay carbon positions.Even at low doping levels, the electronic structure of g-C 3 N 4 was signicantly altered, leading to reduced optical band gap energy and increased electrical conductivity. 91urthermore, P-doped g-C 3 N 4 was synthesized via a thermal polymerization method, where the P atoms were successfully introduced into the g-C 3 N 4 lattice, resulting in modied electronic properties and improved suppressions of charge carrier recombination. 92Moreover, a co-condensation approach, without the use of templates, was followed to synthesize Pdoped g-C 3 N 4 nanoowers with in-plane mesopores, where the introduced phosphorus species exhibited strong chemical bonding with neighboring carbon and nitrogen atoms, leading to a forced planar coordination within the carbon nitride framework. 93urthermore, a single-pot pyrolysis method was employed to synthesize sulfur-doped graphitic carbon nitride porous rods (Spg-C 3 N 4 ) by heating a complex of melamine and trithiocyanuric acid at various temperatures. 94The characterization results demonstrated that S-pg-C 3 N 4 exhibited a porous rod structure with a signicantly higher surface area (ranging from 20 to 52 m 2 g −1 ) when compared to bulk g-C 3 N 4 .Additionally, it was observed that the surface area of the S-pg-C 3 N 4 samples increased as the heating temperature was raised. 94On the other hand, the synthesis of oxygen-doped g-C 3 N 4 using a facile H 2 O 2 hydrothermal method was reported. 95XPS analysis revealed the successful doping of oxygen into the g-C 3 N 4 lattice, resulting in the formation of N-C-O bonds, where oxygen atoms were directly bonded to sp 2 -hybridized carbon.Notably, the oxygen doping induced a downshi of the conduction band (CB) minimum by 0.21 eV without altering the valence band (VB) maximum.This oxygen doping-induced modulation of the electronic and band structure of g-C 3 N 4 and led to various benecial effects, including an increase in visible light absorption, extended surface area and enhanced photogenerated separation efficiency. 95Otherwise, using a hydrothermal synthesis, sulfur uoride-doped carbon nitride (F-SCN) was effectively synthesized. 96The incorporation of uorine and sulfur into the carbon nitride lattice resulted in a notable improvement in the photocatalytic performance by enhancing the separation of electron-hole pairs and facilitating efficient charge transfer. 96n the other hand, the g-C 3 N 4 structure has been modied via metal doping. 97-100For example, mesoporous graphiticcarbon-nitride nanosheets doped with zinc ions (Zn-mpg-C 3 N 4 ) were reported. 101The surface area and porosity of g-C 3 N 4 were improved by PEG-1500, whereas the electrical features of the g-C 3 N 4 increased when zinc was incorporated into the g-C 3 N 4 structure.

Modication of g-C 3 N 4 by creating heterojunctions
Heterojunctions in g-C 3 N 4 -based photocatalysts can be classi-ed into several types based on their structural congurations and electronic band alignments, each offering unique advantages and functionalities for photocatalytic applications.Heterojunctions are typically formed by hybridizing g-C 3 N 4 with other materials, e.g., semiconductors or carbon materials, in a composite form.When these materials are nearby in a heterojunction, they maintain their distinct crystal structures and electrical properties.Different types of heterojunctions, such as Type-I, Type-II, p-n junctions, and Z and S schemes, can be used to create these connections.
2.5.1.Modication by creating Type-I and Type-II heterojunctions.The synergistic combination of g-C 3 N 4 with another photocatalyst can give rise to Type I and Type II heterojunctions, which exhibit fascinating electrochemical and optical properties. 102In Type I heterojunctions, the semiconductor with the wider band gap can promote efficient charge separation and migration.Specically, when illuminated, electron-hole pairs can traverse from the VB and CB of the wider band gap semiconductor to the partner semiconductor, leading to enhanced photocatalytic performances. 102,103urthermore, redox processes take place on the photocatalyst with a lower redox potential, modulating the overall photocatalytic activity.This complex interplay between different semiconductors and their band gaps exemplies the potential for advanced applications in photocatalysis.For instance, the creation of customizable heterojunction structures composed of (CoO x ) encapsulated within g-C 3 N 4 using a straightforward one-pot technique under various annealing environments was demonstrated. 103A Type I heterojunction incorporating Co 3 O 4 /g-C 3 N 4 nanotubes was established in an air setting, resulting in the aggregation of Co 3 O 4 ranging from 20 to 80 nm on the nanotube surface.Another study reported the formation of type I and type II g-C 3 N 4 /g-C 3 N 4 heterostructures for the removal of ppb-level NO in air. 102The research ndings highlight the enhanced photocatalytic activity and stability of the g-C 3 N 4 -based heterostructures compared to pristine g-C 3 N 4 alone.The improved performance can be attributed to the promoted charge separation within the heterostructures, leading to more efficient utilization of light energy and enhanced photocatalytic efficiency in NO removal.
Conversely, misalignment of the conduction and valence band boundaries among the two materials results in the creation of Type II heterojunctions, where the two semiconductors are interfaced while one semiconductor has a lower conduction band and the other has a higher valence band.An inherent electric eld that is generated by the energy level movement at the interface may facilitate charge separation and boost charge migration across the junction.The CB potential of g-C 3 N 4 typically around −1.1 eV, signicantly lower than that of many other photocatalysts.Consequently, when exposed to irritation, e − excited in the CB of g-C 3 N 4 can swily move to the CB of a secondary photocatalyst with a greater potential.In parallel, the generated holes will move in the opposite direction.The creation of a Type II junction allows for the spatial separation of photogenerated electrons and holes, which prevents them from recombining and allows them to participate in desired redox reactions efficiently.This separation of charges leads to an increased lifetime of the charge carriers and enhances the photocatalytic activity of the system.Moreover, the band alignment in Type II heterojunctions can promote interfacial charge transfer processes, such as electron or hole transfer from one component to another, further improving the overall photocatalytic efficiency.This synergistic effect between different semiconductor materials in the heterojunction structure enables better utilization of solar energy and enhances the photocatalytic performance of g-C 3 N 4 -based systems.This phenomenon can be validated through specic analytical techniques like steady-state/time-resolved photoluminescence (PL) spectra, photocurrent measurements, and EIS measurements.5][106][107] For instance, various hierarchical heterojunctions of Bi x O y I z /g-C 3 N 4 , such as g-C 3 N 4 / BiOI, g-C 3 N 4 /Bi 4 O 5 I 2 , and g-C 3 N 4 /Bi 5 O 7 I have been successfully developed. 108The g-C 3 N 4 /BiOI is synthesized using a direct precipitation method, while g-C 3 N 4 /Bi 2.5.2.Modication by creating p-n heterojunctions.The formation of a p-n heterojunction involves combining two different semiconductors with p-type and n-type electronic structures.This arrangement leads to a built-in electric eld at the interface, which can promote charge separation and migration, thereby improving the photocatalytic performance of the material.g-C 3 N 4 behaves as an n-type owing to the -NH/ NH 2 groups as electron donors present in its structure.Constructing a p-n heterojunction promotes the separation of electron-hole pairs.The Fermi level of a p-type (EF,p) is near its VB, while that of an n-type (EF,n) is close to its CB.When p-type and n-type contact, electrons transfer from the n-to p-type owing to the Fermi level offset.This results in a positively charged interface for the n-type semiconductor and a negatively charged interface for the p-type semiconductor, creating a builtin electric eld at the contact interface.For instance, p-n CoFe 2 O 4 /g-C 3 N 4 heterojunctions was created using a simple one-pot coprecipitation method. 109The development of the p-n heterojunction and the distinct structure of g-C 3 N 4 facilitated charge separation and electron transfer, resulting in a remarkable enhancement in photocatalytic activity.The presence of an internal electric eld at the junction boosted the accumulation of electrons and holes in the VB of g-C 3 N 4 and the CB of CoFe 2 O 4 .This led to increased separation efficiency and a noticeable reduction in the recombination rate of electronhole pairs.Other p-n heterojunctions, such as CuAl 2 O 4 /g-C 3 N 4 , 110 BiOCl/g-C 3 N 4 , 111 and MgIn 2 S 4 /g-C 3 N 4 (ref.112) have also been reported.
2.5.3.Modication by creating Z-scheme and S-scheme heterojunctions.The Z-scheme heterojunctions were developed to address the limitations of conventional Type-II heterojunctions.In this arrangement, photogenerated electrons from photocatalyst II are transferred to the valence band (VB) of photocatalyst I.This process enhances the separation of charges in the semiconductor without altering the redox potential of the holes in the VB of photocatalyst II and the electrons in photocatalyst I.In the Z-scheme, the electrons and holes in the lower VB and higher CB levels can be utilized for generating reactive oxygen species (ROS).By maintaining the strong oxidative and reductive properties of the electrons and holes, this heterojunction is preferred over Type-II heterojunctions. 113However, some charge recombination between the lower VB and higher CB levels may still occur.In the direct Zscheme, the transfer of electrons from one photocatalyst to another occurs directly through a physical contact or a solidstate interface between the two photocatalysts.This direct transfer of electrons enables efficient separation and utilization of charges for photocatalytic reactions.In the mediator Zscheme, an additional mediator component is introduced between the two photocatalysts to facilitate the transfer of electrons.This mediator component acts as a shuttle, transferring electrons between the two photocatalysts, thus enabling efficient charge separation and reaction enhancement.The mediator Z-scheme provides exibility in controlling and optimizing the electron transfer process in photocatalytic systems.For instance, 2D/2D Z-scheme BiOI-XBr/g-C 3 N 4 with oxygen vacancies (OVs) was successfully fabricated. 114The introduction of OVs promoted visible-light absorption, acting as an electron mediator to accelerate the separation rate of photogenerated carriers in the Z-scheme.The optimal ratio of the heterostructures exhibited a high photodegradation activity for RhB, which was attributed to the synergistic effects of the 2D/2D Zscheme heterostructure and OVs.
It is worthy to mention that metal oxides heterostructures can not only enhance the visible light absorption ability of g-C 3 N 4 due to their unique band structures but also facilitate the separation and transfer of photogenerated electron-hole pairs, as well as improve the stability and reusability of g-C 3 N 4 photocatalysts.The metal oxides act as protective layers, preventing the photocorrosion of g-C 3 N 4 and enhancing its durability under harsh reaction conditions.This is particularly advantageous for long-term applications and practical implementation.The method used to incorporate the metal oxide into g-C 3 N 4 can signicantly impact the distribution and interaction between the two components, which ultimately affects the photocatalytic efficiency.For instance, TiO 2 is a widely favored photocatalyst due to its excellent chemical stability, affordability, and suitable valence band (VB) and conduction band (CB) positions that facilitate redox reactions. 115,116Thus, a highly efficient heterojunction photocatalyst was developed by combining TiO 2 nanotubes with g-C 3 N 4 through a thermal deposition approach. 117In this process, a solution containing 100 mg of TiO 2 nanotubes and 4 mg of g-C 3 N 4 in 20 mL of distilled water was subjected to stirring at 80 °C for 6 hours.The HRTEM analysis conrmed the close attachment between TiO 2 and g-C 3 N 4 , indicating a strong solid interaction and successful formation of the heterojunction. 117In a separate study, an Sscheme heterojunction of mesoporous/macro TiO 2 /g-C 3 N 4 was fabricated using a straightforward chemical vapor deposition technique. 118The research revealed that by adjusting the melamine dosage, the microstructure of the samples could be readily controlled. 118Similarly, ZnO/g-C 3 N 4 photocatalyst, consisting of ZnO loaded onto g-C 3 N 4 , was fabricated using an ex situ crystallization strategy. 119The images revealed that ZnO particles were present on the g-C 3 N 4 layers, distinguishing it from pure g-C 3 N 4 (Fig. 3a and b). 119XPS analysis conrmed the presence of Zn in the modied catalyst, indicating the successful combination of ZnO with g-C 3 N 4 (Fig. 3c).Moreover, coral-like WO 3 /g-C 3 N 4 were fabricated using a wet chemistry strategy, with different mass ratios of WO 3 to g-C 3 N 4 (1 : 1, 1 : 3, and 3 : 1).TEM images revealed that g-C 3 N 4 appeared as ribbonlike sheets, surrounded by plate-like particles of WO 3 . 121The measurements of the crystallographic particle spacing between 0.20 and 0.39 nm suggest the existence of tiny crystalline zones in the g-C 3 N 4 nanosheets.This close contact between g-C 3 N 4 and WO 3 facilitates the good separation of photo-excited carriers. 121Further, TiO 2 /g-C 3 N 4 composites containing 20-50% TiO 2 by weight were fabricated using a hydrothermal process by dispersing TiOSO 4 in DI water, followed by the addition of g-C 3 N 4 and ultrasonication for 30 minutes. 120The mixture was then heated in an autoclave at 180 °C for 4 hours.The resulting powder was dried at 65 °C.XRD patterns of the composites displayed peaks from both g-C 3 N 4 and TiO 2 , with no shiing in the TiO 2 peaks demonstrating that the TiO 2 lattice structure was not impacted by the coupling with g-C 3 N 4 (Fig. 3d).This lack of inuence on the lattice structure is benecial for photocatalytic activity.Moreover, among the composites, 40% TiO 2 /g-C 3 N 4 had the lowest bandgap energy at 2.89 eV (Fig. 3e). 120In another study, MoO 3 /g-C 3 N 4 was fabricated by combining 0.01 g of Mo 2 N with varying quantities of g-C 3 N 4 and the resulting mixtures were subjected to calcination at 350 °C for 240 minutes. 1224][125][126] Metal suldes possess band structures that meet the thermodynamic requirements for water splitting and exhibit improved responses to sunlight due to the formation of a less negatively charged valence band through the (S-3p) orbitals. 127These advantageous properties of metal suldes signicantly contribute to the superior photocatalytic performance of g-C 3 N 4 /metal sulde heterojunction systems. 112,128The incorporation of metal suldes allows for the creation of customizable band structures, thereby providing tangible benets for the desired photocatalytic reaction.In a study, CdS/g-C 3 N 4 core/ shell nanowires were synthesized using a combination of solvothermal and chemisorption methods. 112Transmission electron microscopy (TEM) analysis revealed that g-C 3 N 4 was effectively coated onto CdS nanowires, establishing intimate contact between the two materials.Additionally, the composite exhibited a higher surface area compared to pure CdS. 112In another investigation, a one-step solvothermal strategy was utilized to synthesize ultra-thin g-C 3 N 4 (UCN) and incorporate   NiS onto the surface of ZnIn 2 S 4 (ZIS). 129The resulting ternary compound, NiS/ZnIS/UCN, was designed to possess dual greatspeed charge transfer channels.By combining these materials, the composite achieved improved efficiency in H 2 generation through enhanced charge transfer. 129It is evident from the TEM picture of NiS/ZIS/UCN that some NiS is loaded onto the surface of ZIS and UCN, implying that the heterojunction ternary compound of NiS/ZIS/UCN has been well constructed. 129In another work, a series of CoS 2 /g-C 3 N 4 were fabricated through a photodeposition strategy. 130The size of the CoS 2 species could be adjusted, ranging from single atom to nanometer scale, allowing for control over the photocatalytic features.The synthesis process involved mixing 20 mg of g-C 3 N 4 with a solution containing 1 mL of 15.2 mg mL −1 thiourea aqueous solution, 1 mL of 5 mg mL −1 Co(CH 3 COO) 2 , 4 mL of ultrapure water, and 4 mL of absolute ethanol.The mixture was evacuated to remove air and then irradiated using a 300 W Xenon lamp to facilitate the deposition of CoS 2 onto the g-C 3 N 4 surface. 130In another work, a solvothermal approach was utilized to create a heterostructure photocatalyst made of g-C 3 N 4 /Bi 2 S 3 /CuS. 131rther, NiS/g-C 3 N 4 , CdS/g-C 3 N 4 , and CdS/NiS/g-C 3 N 4 were created via a simple and dependable chemical deposition technique. 126In another study, g-C 3 N 4 was coated with ternary NiCo 2 S 4 using a solvent evaporation technique. 132Whereby, 30 mL of ethanol was used to dissolve sulphide nanoparticles and g-C 3 N 4 nanosheets, and the mixture was then ultrasonicated for 30 minutes to create a homogenous suspension.Subsequently, the solvent evaporated at 70 °C, yielding a ZnCo 2 S 4 /g-C 3 N 4 photocatalyst.The ZnCo 2 S 4 nanoparticles, which are in very near proximity to the 2D g-C 3 N 4 akes, have a median size of around 20 nm, as determined by TEM investigation (Fig. 4a-d).Moreover, EDS analysis, on the other hand, conrmed that C, N, Zn, Co, and S coexist in the composite and that the atomic ratios of Zn, Co, and S are around 1 : 2 : 4, which is in agreement with the ZnCo 2 S 4 theoretical chemical ratio (Fig. 4e). 1324][135][136][137] For instance, Ag 2 CO 3 /g-C 3 N 4 heterojunctions were fabricated using an ultrasonic method, where Ag 2 CO 3 was sonochemically targeted and xed to the g-C 3 N 4 active centers. 135][144][145] The creation of carbon-induced g-C 3 N 4 photocatalysts presents a viable route for sustained improvements in photocatalytic technology as well as renewable carbon materials as an ecologically benign alternative to metal-based materials.7][148][149] For instance, g-C 3 N 4 /GO (graphene oxide)-wrapped melamine sponge (MS) monolith was developed through successful design and fabrication (Fig. 5). 150The g-C 3 N 4 was uniformly distributed on the GO, ensuring efficient utilization of incident light and effective contact with pollutants.By acting as a bridge, GO facilitated the connection between the g-C 3 N 4 and MS components.In another instance, g-C 3 N 4 /GO nanocomposite was synthesized by loading g-C 3 N 4 onto GO using an electrostatic self-assembly approach. 151rthermore, a unique protonated g-C 3 N 4 /GO aerogel (p-CN/ GOA) was synthesized by a direct frozen-drying technique (Fig. 6a). 152The protonating treatment caused a signicant change in the surface electric charge of g-C 3 N 4 , converting it from negative to positive (p-CN), which allowed for powerful selfassembly with the negative surface of GO.This assembly facilitated the transfer of photogenerated charge carriers.The stacking of p-CN blocks, which were several microns in size, were uniformly attached to the GO nanosheet due to the abundant surface functional groups of GO (Fig. 6c).While TEM conrmed the excellent loading of p-CN onto GO (Fig. 6d), providing further evidence of the combination between p-CN and GOA. 152In order to enhance the efficiency of underwater photocatalysis for g-C 3 N 4 , a composite consisting of g-C 3 N 4 and carbon nanotubes (CNT) was fabricated using an in situ solvothermal approach. 153his composite had great surface area and improved light absorption capacity.The ndings demonstrate that CNT and g-C 3 N 4 exhibit good compatibility with each other.The g-C 3 N 4 can grow directly on the surface of CNT, forming a stable composite structure. 153Another study used a straightforward water bath approach to construct g-C 3 N 4 that had been enhanced with carbon nanotubes (CNTs). 154The morphological study showed that two materials were mixed together and that CNTs were wrapped in a lot of g-C 3 N 4 .This mixture promoted the movement of photogenerated electrons and aided in their separation efficiency. 154Further, carbon bers (CF), graphene (GN), and CNTs were introduced to modify g-C 3 N 4 through a solvothermal approach. 155The development morphology of the synthetic composites varied signicantly depending on the utilized carbon substrate as shown in Fig. 7. 155 The poor physicochemical features (e.g., SBET, particle size, pore volume, adsorptive properties, .etc.), the limited photocatalytic catalytic activity, and stability and poor light-harvesting of pristine g-C 3 N 4 are marginally boosted by proper modication and application of modied g-C 3 N 4 .The superior photocatalytic performance of modied g-C 3 N 4 over pristine g-C 3 N 4 is illustrated by various examples shown in Tables 1 and 2.
3 Applications of g-C 3 N 4 based nanocomposites 3.1.Applications in water treatment 3.1.1.Photocatalytic degradation of organic pollutants.3][194][195] As a solution to this issue, g-C 3 N 4 -based nanomaterials have emerged as highly researched photocatalysts for the treatment of wastewater contaminated with diverse pollutants.These nanomaterials offer numerous advantages, particularly effective adsorption and photocatalytic properties.In this context, we will delve deeper into the discussion of several g-C 3 N 4 -based composites employed for the removal of organic pollutants in wastewater treatment.
For instance, heterojunctions of Bi 2 S 3 /g-C 3 N 4 with varying concentrations of Bi 2 S 3 have been developed for the Rhodamine B (RhB) degradation under sunlight. 196The photocatalytic response is moved to the deep visible spectrum by depositing Bi 2 S 3 on g-C 3 N 4 .When exposed to natural solar radiation, the rate of RhB dye breakdown on 10% Bi 2 S 3 /g-C 3 N 4 is four times higher compared to bare g-C 3 N 4 and Bi 2 S 3 alone.This is explained by the fact that Bi 2 S 3 nanoparticles extend optical reactivity under the whole range of natural sunlight, which lowers the rate at which hole-electron pairs recombine, promotes large charge-carrier movement, and ultimately raises photocatalytic efficiency.The decomposition of RhB is primarily impacted by positive holes, radical species, and superoxide radicals.The S-scheme mechanism described the movement of charge carriers (Fig. 8a), as revealed by terephthalic acid PL examinations and radical scavenging tests (Fig. 8b and c). 196rther, the degradation of methylene blue dye (MB) was carried out using MoO 3 /g-C 3 N 4 heterojunction enhanced with biomass carbon dots.In comparison to bulk g-C 3 N 4 , pure MoO 3 , and pure carbon dots, the heterojunction demonstrated a better degradation rate of 67% throughout one hour of simulated sunlight irradiation. 198Under ideal compounding circumstances, the heterojunction between MoO 3 and g-C 3 N 4 was veried, resulting in an enhanced charge transfer rate at the interface. 198o enhance the photocatalytic activity of g-C 3 N 4 , the researchers loaded g-C 3 N 4 with different magnesium salts. 199mong the various samples tested, the MgSO 4 -g-C 3 N 4 composite exhibited the highest efficiency for photocatalytic degradation, achieving a photodynamic parameter of 26.36 × 10 −3 min −1 .Reactive substances including O 2 c − , h + , and cOH that oxidized MB during the photocatalytic degradation process, where the cOH was the most contributing species. 199In another investigation, g-C 3 N 4 was loaded with potassium salts such as KF, KCl, and KBr, resulting in the formation of KX-g-C 3 N 4 (X = F, Cl, and Br). 200Remarkably, KF-g-C 3 N 4 exhibited exceptional performance in the degradation of MB when exposed to visible light.Notably, KF-g-C 3 N 4 effectively suppressed the recombination of holes and electrons, surpassing the photocatalytic activity of KCl-g-C 3 N 4 , KBr-g-C 3 N 4 , and pure g-C 3 N 4 materials. 200On the other hand, bismuth/g-C 3 N 4 nanotubes (BCN) with a porous structure having various bismuth fractions (0.05-0.40 g) were utilized for the RhB degradation of. 197The highest degradation efficiency was observed with the 0.1 BCN sample, which completely degraded RhB within 40 minutes (Fig. 8d).The degradation kinetics followed pseudo-rst-order behavior (Fig. 8e), and the rate constant (k for 0.1 BCN was 0.0644 min −1 ), which was 26.8 times higher than that of pure g-C 3 N 4 (PCN) (Fig. 8f), where the degradation was inhibited in the presence of isopropanol and p-benzoquinone (Fig. 8g). 197Furthermore, by adopting a straightforward impregnation technique, g-C 3 N 4 -TiO 2 nanocomposites with varying weight proportions (1 : 3, 2 : 2, and 3 : 1) were produced.Under UV-visible illumination, the effectiveness of these nanocomposites in MB dye photocatalytic degradation was examined. 201When contrasted with virgin g-C 3 N 4 and different weight percentages of g-C 3 N 4 /TiO 2 , the nanocomposite with a 3 : 1 weight ratio had the highest photocatalytic activity.Because there were fewer TiO 2 nanoparticles deposited on the g-C 3 N 4 nanosheets, the electron-hole pair transport features were improved, which increased the catalytic efficiency.The creation of a Z-scheme system between TiO 2 and g-C 3 N 4 explains the improved photocatalytic behavior. 201n order to create the TiO 2 @g-C 3 N 4 (TCN) core-shell quantum heterojunction, an effective way of polymerizing the quantum-thick g-C 3 N 4 onto the surface of TiO 2 with exposed facets was adopted and applied the obtained nanocomposite to the photocatalytic degradation of tetracycline (TC), as shown in Fig. 9a. 202,205The maximum rate of tetracycline degradation, exhibited using 100 mg TCN nanocomposite photocatalyst, was 2.2 mg min −1 ; that is 36% more than the rate observed in the TiO 2 /g-C 3 N 4 random mixture (TCN(mix)), twice as high as TiO 2 , and 2.3 times higher than pure g-C 3 N 4 .The distinct advantages of the structure of the quantum-thick g-C 3 N 4 shell, the abundance of readily accessible reaction sites, and the compact and consistent contact interface, are what make TCN more photocatalytically active.The notable improvement in the photocurrent responsiveness of TCN electrodes further supports efficient mobility of electrons among TiO 2 and g-C 3 N 4 .The catalyst's stability was veried by TEM analysis and XRD, as shown in Fig. 9b.The principal oxidant species for the successful photocatalytic process, according to the results, are h + and cO 2 − , as shown in Fig. 9c. 202Furthermore, the researchers found that the improved catalytic activity of CuAl 2 O 4 /g-C 3 N 4 for TC photodegradation is mainly due to the signicant separation of charge carriers, as shown by the transient photocurrent response. 110Moreover, the use of g-C 3 N 4 loaded various metals (Na, K, Ca, Mg) has been studied for the degradation of enro-oxacin (ENR). 203The presence of oxygen atoms in the g-C 3 N 4 nanocomposites has been conrmed through XPS, TEM, and FTIR analysis.These added metals, combined with the oxygen atoms, have altered the electronic structures and morphology of the g-C 3 N 4 , resulting in reduced charge recombination and improved light absorption.As a result, g-C 3 N 4 -Na and g-C 3 N 4 -K produced both hydroxyl radicals and superoxide, while g-C 3 N 4 , g-C 3 N 4 -Ca, and g-C 3 N 4 -Mg only produced superoxide radicals (Fig. 9d).In another study, the integration of graphene onto the edges of g-C 3 N 4 enhanced the absorption of photons with energies below the intrinsic bandgap. 206This integration resulted in a broad-spectrum-driven response and facilitated near-eld electron transfer.The strong p-conjugated bondstitched nanostructures between graphene and g-C 3 N 4 were found to effectively capture adsorbed oxygen molecules, leading to the production of cO 2 − , promoting the interaction between pollutant molecules and the photocatalyst NPs. 206Additionally, the incorporation of reduced graphene oxide (rGO) into g-C 3 N 4 greatly enhanced the photocatalytic activity of bisphenol A (BPA) approximately three times at neutral pH to give 99% removal within 60 minutes. 204The synthesized rGO/g-C 3 N 4 nanocomposite exhibited increased electrical conductivity and improved surface area, leading to enhanced separation of electron-hole pairs, as shown in Fig. 9e and g.The positioning of heterocyclic nitrogen p z orbitals in g-C 3 N 4 was shied aer decorating with rGO, facilitating the polarization of charge distribution, and resulting in the formation of active holes that boosted the BPA degradation. 204.1.2.Effects of operational parameters.Developing effective and long-lasting photocatalytic systems requires a thorough examination of the impact of operating parameters on the photocatalytic breakdown of organic pollutants utilizing composites based on g-C 3 N 4 .To optimize the process, it is essential to comprehend how pH, temperature, coexisting pollutants, light intensity, catalyst dose, and pollutant concentration interact.However, it is crucial to remember that depending on the particular pollutant and photocatalyst under investigation, these characteristics may have different effects.
One of the key parameters to consider is the pH of the reaction medium.pH inuences not only the adsorption capacity of the catalyst but also the protolytic equilibria involving the catalyst and the pollutant, as well as the pollutant's solubility. 207,208These factors can signicantly affect the surface charge of the catalyst and the pollutant molecules, thereby impacting their interaction and subsequent degradation efficiency. 209Therefore, determining the optimum pH range is essential to maximize the photocatalytic performance.However, it should be noted that pH optimization is highly dependent on the specic pollutant and composite being used, as different materials may exhibit different pH sensitivities.The photocatalyst shows positive/negative zeta potentials depending on pH, demonstrating that its surface charge varies signicantly with the solution's pH. 210For instance, the researchers investigated the pH impact on the degradation of RhB and MO dyes, using g-C 3 N 4 @NiAl LDH catalyst. 211They found that the catalyst had a point of zero charge (PZC) value of 6.6 where the highest efficiency for degrading MO occurred at a pH of 3, while RhB degradation was most effective at a pH of 10.Since RhB is a positively charged dye, it experiences repulsion when it approaches the positive surface of the catalyst in the presence of free H + ions, leading to lower degradation at pH 3 compared to neutral or basic conditions.Similarly, MO degradation was reduced under basic circumstances by competition and repulsion among the OH − anions and the anionic MO moieties for adsorption on the photocatalyst. 211dditionally, in the photodegradation of trimethoprim (TMP), peroxymonosulfate (PMS) can be activated by Fe-g-C 3 N 4 with various compositions. 212Thus, it was shown that 0.2% Fe-g-C 3 N 4 /2 wt% rGO/PMS greatly increased the TMP degradation rate in the acidic environment (pH = 3), from 61.4% at pH = 6 to almost 100%.On the other hand, at basic pH levels, where TMP existed primarily as an anionic species, the repulsion among the Fe-doped g-C 3 N 4 /rGO composites and TMP hindered its degradation, leading to lower performance. 212Furthermore, g-C 3 N 4 /TiO 2 (PZC = 6.0) exhibited the highest effectiveness in basic and neutral pH conditions, which promoted the interaction between the cationic RhB molecules and the catalyst's negatively charged surface functional moieties at pH > 6, leading to improved photodegradation of RhB, as shown in Fig. 10a. 213Conversely, g-C 3 N 4 /rGO exhibited pH-sensitive photocatalytic performance toward the photocatalytic degradation of the Rh Cationic dye, with a signicantly greater rate of photodegradation at low acidity levels (pH = 1.98). 217The rate of RhB photodegradation dropped markedly as pH increased and reached almost zero at pH $ 7.This pH sensitive behavior was attributed to the promoted electron-transfer, at lower pH, between RhB, H + , and rGO that acted as a good platform for transferring e − through its atomic sheets. 217he weight or loading amount of the catalyst material can impact various aspects of the photocatalytic process, ultimately affecting the degradation efficiency.One of the key aspects inuenced by the weight of the catalyst is surface area. 218ncreasing the weight of the catalyst generally leads to an increase in the available active surface area for pollutant adsorption and subsequent reaction. 209,219,220This can be bene-cial as it provides more sites for catalytic activity, allowing for a higher number of reactive species to be generated.Consequently, the degradation rate of the organic pollutant may increase with increasing catalyst dose.However, it is important to note that there is an optimum weight or loading amount beyond which further increases may not result in proportional enhancements in degradation efficiency. 221This is because excessively high loadings can lead to aggregation or agglomeration of the catalyst particles, reducing the accessible surface area and hindering the photocatalytic process. 209Moreover, high-weight loadings can also cause light scattering or absorption, limiting the penetration of photons and reducing the overall photocatalytic activity.For instance, the photocatalytic performance of TiO 2 /g-C 3 N 4 improved with increasing catalyst doses until the optimal dose was reached due to the enhancement in the available active sites. 222Moreover, the photocatalytic degradation efficiency of MO dye increased with the CdS/g-C 3 N 4 mass; however, beyond the optimal mass the catalyst particles tended to aggregate, resulting in increased light scattering and lowered overall effective surface area, as well as reduced catalytic activity. 214The results presented in Fig. 10b indicate that the optimal dose of CdS/g-C 3 N 4 for achieving the highest photodegradation of MO is 0.5 g L −1 .A similar trend was also observed for the degradation RB in the presence of g-C 3 N 4 /CdO photocatalyst. 223ncreasing the initial pollutant's concentration can lead to a greater number of pollutant molecules available for adsorption onto the catalyst surface. 195,224This can result in improved initial degradation rates, as more pollutant molecules can interact with the generated reactive radicals.Higher pollutant concentrations can also lead to an increased chance of collisions between the target molecules and the photocatalyst, enhancing the overall degradation efficiency.However, it is important to note that there is an optimum concentration beyond which increasing the initial pollutant concentration may not lead to further enhancements in photocatalytic activity.This is primarily due to two factors.Firstly, at high concentrations, the adsorption sites on the catalyst surface may become saturated, hindering further adsorption of the pollutant molecules.This can limit the availability of reactive radicals and decrease the overall degradation efficiency.Secondly, high concentrations of pollutant molecules in the reaction medium can absorb or even scatter the incident light, preventing it from reaching the photocatalyst surfaces effectively. 225Consequently, the generation of electron-hole pairs and the subsequent reactions may be limited, resulting in reduced photocatalytic activity.For instance, the rate of degradation of the rhodamine B and crystal violet (CV) dyes by the zeolite nanorods decorated g-C 3 N 4 nanosheets (H-ZSM-5/g-C 3 N 4 ) was demonstrated in Fig. 10c and d, illustrating the impact of varying starting dye concentrations. 215In this case, a pseudo-rstorder (PFO) kinetic model explained the dye elimination process.The degradation rate was low at a high concentration (20 ppm) owing to light being impeded from reaching the active sites by the high chromaticity dye molecules present in considerable quantities.Other researchers reported a reduction in the dye degradation at higher concentrations owing to competition among hydroxyl ions and organic substances on active sites as well as the distracted light before reaching the catalyst surface. 223imilarly, in studying the effect of loading ZnO/g-C 3 N 4 nanocomposites with aluminum, magnesium, nickel, copper, and silver, on the degradation rate of 50-300 mg L −1 Eriochrome Black T dye (EBT), the results showed a decreased dye degradation efficiency at higher concentrations of the EBT dye. 226he light intensity plays a signicant role in photocatalytic degradation processes as it directly affects the absorption of photons by the catalyst.Higher light intensities provide a greater number of photons, leading to increased electron-hole pair generation and subsequent formation of reactive species, resulting in improved degradation rates. 68However, it is important to note that once a certain light intensity threshold is reached, further increases may not proportionally enhance photocatalytic activity.In fact, excessive light intensities can lead to increased energy consumption without providing substantial benets.Thus, optimizing light intensity is crucial to achieve the optimal photocatalytic performance.Factors such as the source of light, the wavelength, and the type of catalyst used should all be considered when determining the ideal light intensity for a specic photocatalytic system.For instance, the photocatalytic degradation of sulfamethoxazole (SMX) using Fe-UCN's catalyst was greatly affected by the used light intensity.Under 9000, 12 000, and 15 000 lx of LED light intensity, the SMX % removals were 48%, 75%, and 53%, respectively, as shown in Fig. 10e. 216Therefore, while more intense light may provide the catalysts with photons for creating cOH and lower the pollutant's concentration, too much light may actually inhibit photocatalytic activity due to excessive electron consumption, resulting in the accumulation of extra holes on the catalysts, which hinders the photodegradation process. 216he presence of multiple pollutants can lead to either synergistic or inhibitory effects on the degradation process.Synergistic effects occur when the presence of one pollutant enhances the degradation of another pollutant, due to the formation of reactive species or the modication of the degradation pathway.On the other hand, inhibitory effects can occur when the presence of one pollutant hinders the degradation of another pollutant, due to interactions between the pollutants that can compete for reactive species or affect the availability of active sites, thereby reducing the overall degradation efficiency.Therefore, it is crucial to consider the interactions between pollutants in a mixed system when evaluating degradation efficiency.For instance, pCN-N/ZIS Z-scheme heterojunction was evaluated for the synergistic photodegradation of metronidazole (MNZ) and methyl orange (MO). 227The combination of electron-donating groups on MO and MNZ molecules and electron traps on catalyst surfaces, which improves the catalyst's capacity to contact and adsorb pollutants and ultimately improves the catalytic degradation performance. 227Moreover, the degradation of a mixed MB and RhB dye solution was investigated using ZnFe 2 O 4 -g-C 3 N 4 as the photocatalyst with the addition of H 2 O 2 under sunlight illumination.The MB degradation rate was found to be much greater than that of RhB.As shown in Fig. 11a, aer 35 minutes of exposure to sunlight, the maximum removal of MB was 100%, and in the presence of H 2 O 2 , the maximum removal of RhB was 92%. 228Similarly, the synergistic degradation efficiency of g-C 3 N 4 /a-Fe 2 O 3 for the mixed RhB and MB solution was reported. 229At ve cycles, the fabricated catalyst exhibits a high-performance, as shown in Fig. 11b.Conversely, Co 3 O 4 /g-C 3 N 4 nano-heterojunctions were fabricated to degrade a mixture of TC antibiotic and MB dye pollutants, under solar irradiation.The researchers noticed that MB in the mixed solution showed an improved degradation rate (nearly 100% in 120 minutes) than when it was eliminated individually (90% in 120 minutes). 230When compared to the TC antibiotic's solo activity (97% in 180 minutes), the mixture's antibiotic degradation efficiency was slightly lower (78% in 180 minutes).The formation of intimate interfaces with enhanced photophysical properties was attributed to the band bending induced by the p-n nano-heterojunctions, as shown in Fig. 11c  and d. 230 The degradation efficiency of a g-C 3 N 4 -based Ce 2 O 3 / CuO (GCC) ternary nanocomposite was studied for mixed anionic metanil yellow (MY) and cationic MB dyes, under visible light exposure. 231Notably, the ternary GCC nanocomposite exhibited excellent performance, achieving high removal efficiencies for both MY and MB aqueous dyes (94.5% and 90.3%, respectively).This superior performance can be attributed to the optimal amounts of Ce 2 O 3 and CuO present on the g-C 3 N 4 surface, which facilitated the creation of heterojunction surfaces, thereby efficiently reducing the recombination rates of photo-excited charges. 231

Hydrogen production
The production of hydrogen as a clean and sustainable energy source has gained signicant attention in recent years.Several strategies were employed to enhance the photocatalytic activity of g-C 3 N 4 , which is an efficient photocatalyst that utilizes solar energy to split water and produce hydrogen.When excited by photons with energy equal to or higher than its bandgap, g-C 3 N 4 can generate electron-hole pairs that can be involved in a series of reactions to produce hydrogen.The key steps involved in photocatalytic hydrogen production include light absorption, charge, separation, surface reactions and mass transfer. 232,233ne common approach is the modication of g-C 3 N 4 through metal co-catalyst decoration.For instance, platinum (Pt) nanoparticles can be loaded onto g-C 3 N 4 to enhance the hydrogen evolution reaction (HER) kinetics by providing active sites for hydrogen formation.Other transition metals, such as nickel (Ni) and cobalt (Co), have also been utilized as cocatalysts due to their cost-effectiveness and abundance.Thus, the researchers created a combination of suldized bimetallic nickel and platinum decorated g-C 3 N 4 with various Pt masses for the production of H 2 using visible light.They found that the addition of the NiS x electron acceptor in the S-PtNi X /g-C 3 N 4 catalyst resulted in improved performance compared to catalysts without it, such as PtS x or Suldized-g-C 3 N 4 . 234The existence of PtNi X assisted in the correct transmission of charges.The impressive photocatalytic activity of the S-PtNi X /g-C 3 N 4 catalyst, which achieved a rate of 4966 mmol g −1 h −1 , can be attributed to the collaborative effect of NiS X ability to accept electrons and PtNi X superior charge transfer capabilities. 234nother study described single Pt atom co-catalysts embedded on g-C 3 N 4 via a procedure that involves two stages including incipient wetness impregnation and copolymerization. 235During a 4 hours period, the studies conducted with pure g-C 3 N 4 exhibited minimal activity, generating around 12.7 mmol h −1 g −1 .This suggests that subjecting g-C 3 N 4 to visible light resulted in minimal photocatalytic efficiency.In contrast, the photocatalytic hydrogen evolution of g-C 3 N 4 dramatically improved upon the adoption of 0.1-0.3wt% of single Pt atoms as co-catalysts.The photocatalytic H 2 evolution for Pt 0.1 -g-C 3 N 4 , Pt 0.2 -g-C 3 N 4 and Pt 0.3 -g-C 3 N 4 were about 1054.3, 4875.0 and 2932.8 mmol g −1 in 4 h, respectively.The highest rate of hydrogen generation was obtained with 0.2% Pt-based catalyst, due to its highest negative CB location and remarkable capacity to separate and transmit photogenerated charge carriers. 235oreover, a heterojunction consisting of NiS grown on a 2D ultrathin g-C 3 N 4 matrix was constructed for visible lightinduced H 2 generation. 236The presence of the NiS/g-C 3 N 4 resulted in a synergistic impact, effectively enhancing the separation of photo-generated carriers and promoting interfacial charge transfer performance.The rate of H 2 generation using the exfoliated NiS/g-C 3 N 4 catalyst reached 4.2 mmol h −1 g −1 , which is approximately 2.6 times higher compared to bulk C 3 N 4 /NiS. 236The creation of 0-D/2-D heterojunctions using g-C 3 N 4 nanosheets and polyuorene dots (Pdots) (Pdots/g-C 3 N 4 ) was investigated and showed a substantial rise in photocatalytic HER, reaching 929.3 mmol g −1 h −1 with an apparent quantum efficiency of 5.7% at 420 nm. 237he photocatalytic water-splitting capability of Ag 3 PO 4 /g-C 3 N 4 has been studied. 238The nanocomposite band gap energy value of 2.90 eV, suggest that it may be a successful visible lightharvesting composite.According to the research, compared to the electrons in the CB (0.21 eV) of g-C 3 N 4 , the Ag 3 PO 4 electrons in CB (−1.08 eV) showed more potential for reducing water and protons to form H 2 .Similarly, VB holes of g-C 3 N 4 exhibited stronger oxidizing capabilities than those of Ag 3 PO 4 , resulting in the production of $OH radicals.Ag 3 PO 4 /g-C 3 N 4 composite showed an electron transformation mechanism that resulted in the production of a Z-scheme process, which is benecial for water splitting to produce H 2 . 238sing Ti 3 C 2 MXene as a precursor, carbon-doped TiO 2 (C-TiO 2 ) linked with g-C 3 N 4 was synthesized. 239In comparison to pure TiO 2 with average particle size of 25 nm (P25), the C-TiO 2 exhibited a lowered bandgap of 2.94 eV, implying boosted visible light absorption with a redshied absorption edge at 425 nm.As a result, the 10% C-TiO 2 /g-C 3 N 4 catalyst produced hydrogen at a rate of 1409 mmol g −1 h −1 (l > 420 nm) with enhanced activity ascribed to the creation of a Type II heterojunction, which enables optimum charge separation and increased accessorial surface area, offering extra reaction sites upon coupling with C-TiO 2 . 239oreover, the researchers applied FeO x /g-C 3 N 4 for the improved efficiency H 2 evolution through water splitting. 240The optimized amount of FeO x led to an impressive H 2 evolution rate of 108 mmol h −1 that is 4.2 times higher than that of pristine g-C 3 N 4 .Numerous reasons, such as increased surface area, greater electron transfer ability, better visible light absorption, and superior charge carrier separation, are responsible for this improvement. 239ther researchers conducted a study on the fabrication of g-C 3 N 4 /CNTs for achieving high-efficiency H 2 production. 241They incorporated different types of CNTs, including single-walled (SW), double-walled (D), and multi-walled (MW), to enhance the activity of g-C 3 N 4 -based photocatalysts.Enhanced production of photocatalytic hydrogen was seen when the amount of CNTs is low, leading to a boost in the stability and quantity of photogenerated charges.The improved electron transport from g-C 3 N 4 to CNTs, which was particularly apparent in the case of SWCNTs, accounts for this improvement. 241dditionally, the S-scheme heterojunction of N-doped MoS 2 / S-doped g-C 3 N 4 was successfully constructed using a straightforward one-step thermal polymerization approach. 242ollowing material optimization, the catalyst's photocatalytic hydrogen generation rate reached 658.5 mmol g −1 h −1 .This was made possible by the boost in visible light absorption and photogenerated carrier separation yield caused by the Sscheme's design.
Further, a comprehensive investigation on the impact of three common transition metal phosphides (M 2 P, M = Fe, Co, and Ni) as cocatalysts in sulfur-doped g-C 3 N 4 (S-CN) was investigated. 243The researchers utilized an ultrasound-assisted approach to create M 2 P/S-CN with similar load ratios, ensuring comparable crystallization levels and particle sizes.Ni 2 P/S-CN demonstrated the most rapid charge transfer and separation among the three phosphides, resulting in smaller photocatalytic overpotential.This remarkable performance yielded a rate of hydrogen generation that was comparable to that of Pt/ S-CN catalysts and 22.7 times higher than that of bare S-CN. 243therwise, a simple wet-chemical fabrication approach was used to successfully produce a dual Z-scheme heterostructure of g-C 3 N 4 , PrFeO 3 , and Fe 2 O 3 . 244This cascade dual Z-scheme exhibited impressive production, generating 379.29 mmol g −1 h −1 under visible-light exposure.The inclusion of magnetic components in the heterostructure facilitated the easy separation of the catalyst and enabled its reusability.Additionally, RuNi/g-C 3 N 4 catalysts doped with 2D bimetallic RuNi alloys were created using the solvothermal deposition approach involving various Ru ratios.The catalyst sample having 2.3 wt% Ru revealed the greatest hydrogen evolution, reaching 35 100 mmol g −1 h −1 , surpassing the performance of the Pt/g-C 3 N 4 photo-catalyst. 245Table 3 shows the photocatalytic H 2 -evolution performance characteristics of representative g-C 3 N 4 -based photocatalysts.

Carbon dioxide reduction
Applying g-C 3 N 4 -based nanocomposites for CO 2 reduction holds signicant promise for addressing the global challenge of climate change by transforming CO 2 emissions into valuable products, such as methane, methanol, and hydrocarbons.The photocatalytic activity of g-C 3 N 4 -based composites is attributed to their unique structure and composition, which facilitate the absorption of light and generation of reactive species for CO 2 sequestration, contributing to the reduction of greenhouse gas emissions and the development of a circular carbon economy. 256Fig. 12 depicts the basic steps involved in CO 2 photoreduction involving surface and optoelectronic properties. 257-C 3 N 4 based photocatalysts play a robust role in the process of CO 2 photoreduction through their optoelectronic and physicochemical features.These catalysts expose active sites on their surfaces where CO 2 adsorption and activation take place.Therefore, when designing C 3 N 4 -based photocatalysts, it is essential to prioritize factors such as efficient visible light absorption, promote surface area, quick electron transfer to the catalyst surface, exposure of functional groups, minimized recombination rate, and a robust redox potential value.Fig. 13 shows how CO 2 is transformed into methane and methanol on the surfaces of g-C 3 N 4 .258 The process starts by capturing and activating CO 2 when two electrons are generated.257 Then, an intermediate called COOH* is produced, which eventually converts into CO.The hydrogenation of CO* into COH* or CHO* is a signicant step in CO 2 reduction.258 For instance, the Ag 3 PO 4 @g-C 3 N 4 hybrid promoted the photocatalytic reduction of CO 2 .259 This was achieved by forming a heterojunction structure between Ag 3 PO 4 and g-C 3 N 4 , which promoted the CO 2 reduction activity through a Z-scheme mechanism that facilitated the charge separation phenomena.When exposed to simulated sunlight, the optimized Ag 3 PO 4 @g-C 3 N 4 hybrid demonstrated a robust CO 2 conversion rate of 57.5 mmol h −1   , surpassing the rates of pure g-C 3 N 4 and P25 catalysts by 6.1 and 10.4 times, respectively.Further, graphene-supported 1D nano-arrays of crystalline carbon nitride (1D-CCN) heterojunction was developed and demonstrated promoted interface charge transfer, facilitated light absorption, and promoted CO 2 capture capabilities.260 Furthermore, the 1D-CCN demonstrated a 44% selectivity for CO 2 over N 2 , with isosteric heat adsorption of 55.2 kJ mol −1 for CO.
1][262][263] Among these dopants, O-and P-doped g-C 3 N 4 demonstrated robust conversion capabilities compared to pure g-C 3 N 4 .Additionally, S-doping and creating N-vacancies can introduce impurities in the conduction band position of g-C 3 N 4 , expanding light absorption to longer wavelengths and minimizing recombination rates.Moreover, the researchers created ternary hybrids (ACNNG-x) by combining AgBr with g-C 3 N 4 -modied nitrogen-doped graphene (NG) in various ratios. 264These catalysts were employed for reducing CO 2 using visible light.The process of making the composite and SEM image of the optimized ternary hybrid are displayed in Fig. 14a and b, respectively.The optimized ternary composite demonstrated promising CO 2 reduction rates of 105.89 mmol g −1 for methanol and 256.45 mmol g −1 for ethanol.A proposed mechanism for the process is presented in Fig. 14c.Similarly, g-C 3 N 4 /NaNbO 3 nanowires were synthesized for CO 2 reduction. 265nhancing the overall system performance by modifying g-C 3 N 4 with a component for CO 2 adsorption has proven effective.For instance, g-C 3 N 4 combined with a cobalt-containing zeolitic imidazole framework (Co-ZIF-9), demonstrated high CO 2 adsorption capacity (2.7 mmol g −1 ) and a signicant microporous surface area (1607 m 2 g −1 ), facilitating CO 2 capture and concentration in its pores. 266The addition of bipyridine as an electron mediator allowed photoexcited electrons to transfer from g-C 3 N 4 to Co-ZIF-9 for CO 2 reduction, as shown in a PL quenching study.In this system, CO was the primary product, achieving a quantum efficiency of 0.9% without the need for a cocatalyst. 266Moreover, g-C 3 N 4 /Bi 2 WO 6 hybrid was hydrothermally fabricated to selectively convert CO 2 to CO through photoreduction. 267The hybrid demonstrated a visible-light CO generation rate of 5.19 mmol g −1 h −1 , surpassing that of g-C 3 N 4 alone.The hybrid's improved photocatalytic activity was attributed to effective charge separation and transfer following a Z-scheme mechanism. 267 272 Where n S-scheme heterojunction promoted the light absorption and the separation of photogenerated charges, resulting in enhanced photocatalytic performance and improved H 2 O 2 production. 273Additionally, pairing of g-C 3 N 4 with other materials, such as GO and metalorganic frameworks (MOFs) further improved the H 2 O 2 production efficiency. 274,275n the case of g-C 3 N 4 /GO composites, the GO acts as an efficient electron acceptor, facilitating the separation of photogenerated electron-hole pairs in the composite. 191The high specic surface area and excellent electrical conductivity of GO contribute to the improved H 2 O 2 production efficiency.Similarly, the integration of g-C 3 N 4 with MOFs can provide a high surface area and tunable pore structure, enhancing the adsorption of reactants and the photocatalytic H 2 O 2 production. 276he choice of materials and their relative positioning within the composite can synergistically impact the light absorption, charge separation and transfer processes and the overall catalytic activity.Computational studies using density functional theory (DFT) calculations have provided valuable insights into the electronic structure, band alignment, and charge carrier dynamics at the g-heterostructure interfaces. 276,277  calculations and experimental data attributed the boosted photocatalytic activity of the modied heterostructure to the positively charged MOF sheet interlayer, and the coupling between MOF nanosheet, g-C 3 N 4 , and CuO that can enrich ions, electrons, and molecules and obstruct holes to greatly boost the rapid separation of photogenerated carriers from g-C 3 N 4 and/or CuO, and the reactants' adsorption. 276Thus, incorporating suitable metal active centers into the g-C 3 N 4 framework is an effective approach to promote the activity and selectivity of the oxygen reduction reaction (O 2 RR). 278The adsorption of O 2 on the metal surface can occur in three different congurations: Griffiths-type (side-on), Pauling-type (end-on), and Yeager-type (side-on). 269For instance, the researchers developed a novel Sb-single-atom photocatalyst (Sb-SAPC) doped g-C 3 N 4 that exhibited exceptional performance, generating H 2 O 2 at 12.4 mg L −1 , 248 times higher than pristine g-C 3 N 4 . 279The enhanced activity of Sb-SAPC-g-C 3 N 4 is attributed to the Sb-SAPC sites that facilitated O 2 adsorption and activation, where the accumulation of photogenerated holes at neighboring Natoms near Sb sites promotes the oxygen evolution reaction (OER) for O 2 production.The Sb-OOH intermediates suggest a direct one-step, two-electron reduction pathway for H 2 O 2 generation. 279orming a heterojunction structure is a successful approach to addressing the challenge of charge carrier recombination in pristine g-C 3 N 4 .This is because the difference in Fermi level between g-C 3 N 4 and the coupled co-catalysts drives the photogenerated charge carriers to migrate between the two components.For instance, a 2D/2D heterojunction composed of ZnIn 2 S 4 and g-C 3 N 4 (ZIS/CN) was prepared employing a simple oil bath heating approach. 280The obtained data demonstrated that the H 2 O 2 production proceeded through a 2-electron oxygen reduction (2e − O 2 RR), reecting a robust selectivity towards H 2 O 2 generation.The promoted photocatalytic performance was attributed to the synergistic impact of intimate interfacial contact.In another study, an oxygen-doped g-C 3 N 4 modied g-C 3 N 4 /TiO 2 (OCN@CNT-2) hybrid system was constructed through an electrostatic self-assembly approach, where a double Z-scheme architecture was formed within the target OCN@CNT-2 composite. 272This unique heterojunction promotes the charge separation under the inuence of an internal electric eld.As a result, aer 60 minutes, the system was able to achieve a remarkably high H 2 O 2 yield of up to 133.04 mmol L −1 .
The produced H 2 O 2 can nd various applications, including water purication, disinfection, and oxidation processes.Ongoing research aims to further improve the H 2 O 2 production efficiency, stability, and scalability of g-C 3 N 4 -based composite photocatalysts, paving the way for their practical implementation in large-scale H 2 O 2 production systems.

Conclusion and prospective
In conclusion, this comprehensive review article has covered various aspects of g-C 3 N 4 based nanocomposites, including their synthesis and characterization methods, their application in the removal of organic pollutants and hydrogen production, and the factors inuencing their photocatalytic activities.Through the incorporation of dopants, metal deposition, metal chalcogenide semiconductors, and carbon materials, these nanocomposites have exhibited remarkable photocatalytic capabilities with potential for real-world environmental remediation and energy production.The synthesis and characterization techniques discussed in this article have provided valuable insights into enhancing the performance and stability of g-C 3 N 4 -based composites.The introduction of dopants and metal deposition, as well as metal chalcogenide semiconductors have enabled the modication of the band structure and surface properties, thereby improving the separation and transfer of photogenerated charge carriers.The incorporation of carbon materials, such as graphene or carbon nanotubes, has contributed to the enhancement of photocatalytic activity by increasing the surface area and facilitating electron transfer.The photocatalytic degradation of various organic pollutants, including dyes, pesticides, and pharmaceutical compounds, has been effectively achieved using g-C 3 N 4 based composites.Additionally, the production of hydrogen as a clean and sustainable energy source has been successfully demonstrated through photocatalytic water splitting.The investigation of factors affecting the photocatalytic process has deepened our understanding of the mechanisms involved and has highlighted the important working factors such as catalyst dose, pH, and light intensity.This knowledge can be utilized to optimize the design of g-C 3 N 4 based nanocomposites, tailoring them for specic applications and improving their overall performance and efficiency.
Looking to the future, there are several exciting prospectives for further development in the eld.
1.The scale-up of synthesis methods and the development of cost-effective production techniques are essential for the practical application of g-C 3 N 4 -based composites.Efforts should also be made to evaluate their long-term stability and recyclability to ensure their viability for laboratory, pilot-plant and large-scale implementation with the involvement of engineering and chemistry disciplines.
2. In the pursuit of constructing novel g-C 3 N 4 -based photocatalysts, there is a need for template-free and environmentally friendly synthetic approaches that can yield unique structures and exceptional intrinsic properties.However, the current methods of modifying these photocatalysts have certain limitations.Some of the selected composite materials contain expensive and environmentally detrimental elements.Achieving precise chemical doping of g-C 3 N 4 is a difficult task that oen results in the introduction of impurities.Furthermore, the available techniques for controlling the structure of g-C 3 N 4 are relatively limited and have only minimal effects.Additionally, achieving precise control over the microstructure of these photocatalysts remains a challenging endeavor.
3.More detailed and specic reporting is needed to elucidate the synergistic effects that occur among the individual materials in complex heterostructures.
4. While there is a theoretical understanding of the charge transfer and separation pathways, further experimental evidence is necessary to validate these photochemical mechanisms and establish effective photocatalytic systems on a larger scale.
5. In the realm of photocatalytic degradation, it is crucial to address the simultaneous degradation several pollutants present in real wastewater using g-C 3 N 4 -based materials.Furthermore, g-C 3 N 4 -based photocatalysts hold signicant potential for bifunctional catalysis, considering their catalytic economy and efficiency.
6. Furthermore, it is crucial to preserve and enhance the biocompatibility and eco-friendly properties of future g-C 3 N 4based nanomaterials.
7. To meet the industrial aim of photocatalytic hydrogen production, the solar to hydrogen (STH) efficiency must be at least 10%.Currently, the maximum efficiency attained in laboratory research is 9.2%, while the STH efficiency for g-C 3 N 4 is less than 3%, indicating that much more work remains to be done.The most signicant job for the g-C 3 N 4 photocatalyst is to construct more efficient electron transport systems.
8. Gaining a comprehensive understanding of the underlying mechanisms driving photocatalytic H 2 O 2 production is essential.Researchers should direct their efforts towards meticulously analyzing the various factors inuencing this process, such as the adsorption dynamics of O 2 , the impact of the catalyst's surface properties on the adsorption and activation of O 2 , the intermediate stages involved in H 2 O 2 generation, and the role of active species in modulating H 2 O 2 production.
Despite the challenges mentioned, with continued efforts, g-C 3 N 4 -based materials still can hold great potential and limitless opportunities for large-scale environmental applications.

Data availability
The data analyzed in this review article are from previously published studies.The specic datasets and sources are cited throughout the manuscript and listed in the reference section.Readers can access the underlying data from the original published sources as cited.The authors conrm that they did not have any special access privileges to these datasets.The data analyzed in this review article are from previously published studies.The specic datasets and sources are cited throughout the manuscript and listed in the reference section.Readers can access the underlying data from the original published sources as cited.The authors conrm that they did not have any special access privileges to these datasets.

Fig. 2
Fig. 2 Tem image of (a) g-C 3 N 4 , (b) Au/g-C 3 N 4 , (c) DRS data of g-C 3 N 4 nanosheets, bulk g-C 3 N 4 , and AuNP/g-C 3 N 4 nanohybrids reprinted with the permission of ref. 72, copyright 2024, American Chemical Society; (d) synthesis of Ag/g-C 3 N 4 via green route, reprinted with the permission of ref. 73, copyright 2024, Elsevier; and (e) EIS of the g-C 3 N 4 , and pure and Fe-doped g-C 3 N 4 nanosheets, reprinted with the permission of ref. 74, copyright 2024, RSC.
4 O 5 I 2 and g-C 3 N 4 /Bi 5 O 7 I are obtained through in situ calcination transformation of g-C 3 N 4 /BiOI at different temperatures.The g-C 3 N 4 /BiOI and g-C 3 N 4 /Bi 4 O 5 I 2 heterojunctions are classied as Type-I, while g-C 3 N 4 /Bi 5 O 7 I is categorized as a Type-II heterojunction.Notably, g-C 3 N 4 /Bi 5 O 7 I exhibited signicantly improved performance compared to g-C 3 N 4 /BiOI and g-C 3 N 4 /Bi 4 O 5 I 2 .The promoted activity of g-C 3 N 4 /Bi 5 O 7 I can be attributed to its surface area, promote charge separation and transfer performance, and robust charge carrier density resulting from the formation of a Type-II heterojunction.

Fig. 5
Fig. 5 (a) Schematic illustration of the preparation of g-C 3 N 4 /GO-wrapped sponge; (B): image of different shapes of g-C 3 N 4 /GO-wrapped sponge, reprinted with the permission of ref. 150, copyright 2024, Elsevier.

Fig. 6
Fig. 6 (a) Schematic of the fabrication of p-CN/GOA; (b) zeta potential of GO, g-C 3 N 4 and p-CN, (c) the SEM of p-CN/GOA; (d) the TEM of p-CN/GOA, reprinted with the permission of ref. 152, copyright 2024, Elsevier.

Fig. 8
Fig. 8 (a) An S-scheme for charge transfer between g-C 3 N 4 and Bi 2 S 3 in CNBiS10 catalyst, (b) PL of terephthalic acid over CNBiS 10 , and (c) effect of various scavengers on photocatalytic removal of terephthalic acid over CNBiS 10 reprinted with the permission of ref. 196, copyright 2024, Elsevier; (d) degradation profiles of RhB catalyzed by polymeric carbon nitride (PCN) and Bi doped g-C 3 N 4 at various ratios (BCN), (e) Pseudo firstorder kinetics curves of RhB degradation, (f) apparent rate constant histogram of RhB, and (g) degradation of RhB with various radical quenchers reprinted with the permission of ref. 197, copyright 2024, RSC.

Fig. 9
Fig. 9 (a) Degradation of tetracycline by TiO 2 /g-C 3 N 4 (TCN), 202 (b) XRD patterns of TCN before and after TC degradation process, TEM image (inset) of the used TCN, (c) effect of different scavenger on TCN photocatalytic activity reprinted with the permission of ref. 202, copyright 2024, Elsevier; (d) degradation mechanism of enrofloxacin (ENR) by g-C 3 N 4 , in the absence and the presence of Na, K, Ca, and Mg dopants reprinted with the permission of ref. 203, copyright 2024, Elsevier; (e) EIS measurements presented as the Mott-Schottky plot, 204 (f) the Nyquist plot, and (g) the photoluminescence spectra of bisphenol A photodegradation in the presence of rGO/g-C 3 N 4 nanocomposites with different rGO ratios reprinted with the permission of ref. 204, copyright 2024, Elsevier.

Fig. 10 (
Fig. 10 (a) Effect of pH on degradation of RhB dye reprinted with the permission of ref. 213, copyright 2024, Elsevier, (b) the effect of catalyst mass on degradation of MO dye, reprinted with the permission of ref. 214, copyright 2024, Elsevier; (c and d) effect of initial RhB and CV dye concertation on the degradation performance reprinted with the permission of ref. 215, copyright 2024, Elsevier; (e) effect of light intensity on sulfamethoxazole removal, reprinted with the permission of ref. 216, copyright 2024, Elsevier.

Fig. 11 (
Fig. 11 (a) UV absorption spectrum of Mixed dye (MB + RhB) by ZnFe 2 O 4 -CN, 228 (b), recycling catalytic activity measurement for mixed pollutants, 229 (c) band alignment of p-type Co 3 O 4 and n-type C 3 N 4 before junction formation and (d) band alignment and the photocatalytic mechanism of Co 3 O 4 -C 3 N 4 p-n nano-heterojunctions, reprinted with the permission of ref. 230, copyright 2024, RSC.

Fig. 13
Fig. 13 Proposed reaction pathway for CO 2 reduction to methanol and methane on the surface of g-C 3 N 4, reprinted with the permission of ref. 258, copyright 2024, Elsevier. DFT

Fig. 14 (
Fig. 14 (a) Synthesis process of ACNNG-x hybrid, and (b) its SEM image, and (c) a mechanism for CO 2 reduction by the hybrid nanocomposites, reprinted with the permission of ref. 264, copyright 2024, Elsevier.

Fig. 15 A
Fig. 15 A summary of recent H 2 O 2 -generation methods based on g-C 3 N 4 photocatalysts. 269 A: Mahmoud

Table 2
Photocatalytic degradation performance of various g-C 3 N 4 based ternary photocatalysts

Table 3
Photocatalytic H 2evolution performance of various g-C 3 N 4 -based photocatalysts